American senators visiting Iraq warned the Baghdad government Wednesday that it risked damaging relations with the U.S. if it is allowing Iran to fly over its airspace to deliver weapons to Syria.

An Iraqi government spokesman responded by saying Iran has told Baghdad the flights to Syria are only delivering humanitarian aid. He said the onus is on the U.S. to offer up proof that Tehran is shipping weapons.

Senator Joe Lieberman, an Independent from Connecticut, said Iraq’s failure to stop the flights could threaten the long-term relationship with the U.S. as well as aid Iraq could receive as part of a 2008 strategic pact between the two nations.

“Bottom line, this kind of problem with these Iranian overflights can make it more difficult to proceed with the Strategic Framework Agreement in the manner that the prime minister and we would like to see happen,” Lieberman told reporters in Baghdad. “So I hope this is cleared up quickly.”

Iran is Syria’s closest ally in the Middle East and it has stood by President Bashar Assad as his forces have tried to crush the uprising there for the past 17 months. Activists say at least 23,000 have been killed.

The dispute over the flights was first reported in The New York Times on Wednesday. It said American officials believe Iran resumed shipments of military equipment to Syria via Iraqi airspace in July after a three-month hiatus.

“This region is about to explode,” said Senator Lindsey Graham, a Republican from South Carolina who was visiting Iraq with Lieberman and Republican John McCain of Arizona. “They’re in a pickle here,” Graham said at the same meeting with reporters where Lieberman spoke.

“The reason they’re probably not pushing back on Iran is because they don’t see how this ends. There’s an amazing lack of American leadership, and it’s beginning to show on all fronts,” Graham added.

The three hawkish senators said the dispute was one example of how U.S. influence is dwindling in Iraq even after it pumped in billions of dollars of aid and fought a nine-year war that ousted the dictatorship of Saddam Hussein and cost nearly 4,500 American lives.

Ali al-Moussawi, media adviser to Iraqi Prime Minister Nouri al-Maliki, confirmed that Iranian planes are flying over Iraq to deliver goods to Syria. But he said Tehran has assured al-Maliki that the flights are carrying only food and other humanitarian aid to help victims of Syria’s civil war.

Iraqi and American officials say the U.S. believes otherwise. They said U.S. Vice President Joe Biden has promised to send al-Maliki evidence that the flights contain weapons to help Assad’s regime – something that would be a violation of a U.N. Security Council agreement.

“The Iraqi government is carefully monitoring this issue both in the sky and ground,” al-Moussawi told The Associated Press. He said Iraq has warned Iran against flying weapons though its airspace.

“The Iranian government has said that it respects our decisions,” he insisted. “Until now, there is no evidence of any violation in this regard, and if anyone has any evidence, they should bring it to us and we will take the needed measures,” al-Moussawi said.

He said Biden promised to show al-Maliki proof of the weapons shipments about a month ago – but claimed the Iraqis never received it.

Tommy Vietor, the National Security Council spokesman at the White House, did not immediately respond when asked if Biden promised to give proof of the shipments. He said Tehran “will stop at nothing to support a Syrian regime that is murdering its own people,” adding that Iraq, like all other nations, must block Iran from exporting arms.

“Iraq has taken steps in the past to meet that obligation and it must continue to do so,” Vietor said in an email. “We expect Iraq, as a member in good standing of the international community and a strategic partner of the United States, to meet its international obligations.”

Iraq has insisted it is not taking sides in the uprising. But its Shiite-led government has been trying to foster closer ties with Shiite Iran over the last several years, even as western powers have demanded that Assad step down.

It’s not clear how Iraq would stop Iran from using its airspace in flying weapons to Syria. Iraq’s air force has only a small number of planes, and no fighter jets to protect its skies or enforce its sovereignty. That mission was previously handled by the U.S. military before American troops withdrew from Iraq last year.

At the Pentagon, two senior military officials said the best way for Iraq to confirm what’s on the flights is for the Baghdad government to have the Iranian flights land and to inspect the cargo. They spoke on condition of anonymity because they were not authorized to discuss the issue on the record.